Current piston ring assemblies in internal combustion engines have very high friction. Substantial fuel economy gains are available if it is possible to reduce this operating friction. Calculation indicates that piston ring friction would be cut by more than a factor of 10 if the piston rings functioned as optimally designed bearings. Currently, ring assemblies operate with very thin oil films, largely because the ring assemblies include oil scrapers which scrape the oil layer on the cylinder wall down to extremely thin films.
It is worthwhile to review the fundamentals required for design of a low friction piston ring assembly. Sliding parts operating with full oil films have much lower friction than parts sliding in the boundary lubrication regime. The friction for a full film is inverse with film thickness. There is, therefore, a strong incentive to produce the thickest possible oil films between sliding parts. Thick, low friction oil films are set up by wedge film geometries. The basic equation for oil wedges is Reynold's equation, which is explained thoroughly in a number of textbooks and references (i.e. p. 3-4 Standard Handbook of Lubrication Engineering, O'Connor and Boyd, McGraw-Hill, New York, 1968). The application of Reynold's equation is well understood in cases that involve fully convergent wedges. In cases involving geometries with both convergent and divergent sections the equation is easily applied for heavily loaded cases. Hydrodynamic lubrication has been exhaustively treated in the literature. It is an area of mechanical engineering where exact mathematical equations are known to work within any reasonable experimental error.
The convergent angles and geometrical relations of hydrodynamic film physics involve very critical geometrical issues. The heavy loads occurring in engines invariably deform the parts in a way which affects the film forming geometry and the friction. The film thicknesses and geometrical relations required for optimal full-film physics often require geometrical precision not reasonably obtainable in production. Moreover, heavy loads produce deformations such that the oil film forming geometry varies from the geometries which would occur if the engine parts were infinitely stiff. Differential expansion of parts also alters film forming geometry. It is a purpose of the present invention to present piston ring designs where the rings conform elastically. The ring deformations automatically adjust geometry to maximize oil film stability and minimize friction. Optimization of flexibility of the rings permits fine scale adjustment of film forming geometry which is impossible with production tolerances alone, particularly when differential expansion of parts is considered.
The flexible rings are mounted in circumferential compression with end-gap springs. This elastic arrangement permits the rings to follow cylinder bore out-of-round distortions as the piston moves. The ring pack is arranged with each ring crowned so that its interaction with the cylinder wall oil film is essentially cylinder on flat plate fluid mechanics. The combination of spring forces, gas actuating forces, and radii of crown curvature produces a situation where the bottom ring, called the oil control ring, is always fully flooded and the other rings are operated in a partially starved but fully hydrodynamic condition. The rings should have essentially zero wear and extremely low friction. The ring assembly does not have a conventional scraper. This allows fully flooded lubrication of the piston skirts. In the place in the piston where an oil scraper might ordinarily be is a groove functioning as an oil reservoir to assure that the cylinder wall and ring pack stay flooded.
It is desirable to have a ring package which functions with full film lubrication at all times. This requires thicker cylinder wall oil films than now occur. It is expected that with the combination of a smooth cylinder wall and the disclosed rings, oil control can be achieved. However, for some applications it may be useful to modify the cylinder wall near the top dead center positon of the top ring with a large number of shallow axial grooves, which will serve as compression blowdown paths to actively blow oil down from the top of the cylinder for oil control. This is an assured way to get oil control, and permits the rings to operate with thicker oil films than would otherwise be practical.